Relict landscape evolution and fault reactivation in the eastern Tian Shan: Insights from the Harlik Mountains

Abstract

Relict low-relief surfaces, formed during tectonically quiescent periods and later modified by factors such as increased tectonic activity, are prevalent within the active mountain ranges of central Asia. However, their formation, preservation, and subsequent evolution within the Mesozoic-Cenozoic tectonic framework remain poorly understood. This study examines the low-relief surfaces of the Harlik Mountains, located in the easternmost Tian Shan, integrating digital terrain analysis, fluvial geomorphic analysis, structural geology, and low-temperature thermochronology to reconstruct their long-term geomorphic evolution. Our results reveal that these surfaces are segmented by WNW-ESE-striking faults, which initially experienced right-lateral transtensional movement followed by left-lateral strike-slip reactivation. Apatite fission-track (AFT) thermochronology of samples from relict surfaces yields AFT ages ranging from 1/4110 to 1/4100Ma, while samples from fault zones record ages of 90-70 Ma. Thermal modeling of these samples indicates a period of moderate cooling in the mid- to late Early Cretaceous, followed by a prolonged slow cooling phase for the relict surfaces. In contrast, fault zones show rapid cooling during the 90-70 Ma interval. By integrating these results with previous findings, we propose that the mid- to late Early Cretaceous (1/4110-100Ma) cooling event corresponds to extensional collapse following building of the Mongol-Okhotsk orogen. This process, coupled with increased humidity, enhanced erosion, and relief reduction, facilitated the formation of low-relief surfaces. The influence of Mongol-Okhotsk orogenic collapse likely persisted into the Late Cretaceous (90-70 Ma), during which right-lateral transtensional faulting further segmented the landscape without generating significant topographic contrasts. By the Oligocene (1/430Ma), far-field effects from the India-Eurasia collision reactivated major faults in a left-lateral sense, driving regional uplift, surface tilting, and drainage incision. This uplift phase marked the end of landscape stability, as evidenced by increased sediment input into adjacent basins. Despite active faulting and fluvial incision, generally low erosion rates allowed the preservation of large-scale Mesozoic low-relief surfaces. Copyright: